General background on the Long Term Evolution (LTE) is given in the following subsection. Additional information can be obtained from the 3rd Generation Partnership Project (3GPP) LTE specifications, including in particular the specifications 3GPP TS 36.211 V11.3.0 (“Physical Channel and Modulation”), 3GPP TS 36.212 V11.3.0 (“Multiplexing and Channel Coding”), 3GPP TS 36.213 V11.3.0 (“Physical Layer Procedures”), and 3GPP TS 36.300, V11.6.0 (“Overall Description”).
Densification and Heterogeneous Deployments
In order to meet higher capacity demands and enhanced user experiences, cellular communications networks, such as LTE, need to be deployed with an increasing density of base stations. This densification can be achieved by cell splitting of macro cells and by deploying small cells in highly loaded geographical areas, or so called traffic hotspots, within the coverage area of macro cells. With densification of cellular networks, radio resources can be further reused and users will come closer to the serving base station which enables higher bitrates. Cellular networks with a mixture of macro cells and small cells with overlapping coverage areas are sometimes referred to as heterogeneous networks which are seen as an important complement to macro cell splitting. One example of such deployments is where clusters of pico cells are deployed within the macro coverage area to offload macro traffic. A pico base station represents here one example of a Low Power Node (LPN) transmitting with low output power and by then covers a much smaller geographical area than a high power node like a macro base station. Other examples of low power nodes are home base stations and relays.
A consequence of network densification is that User Equipment (UE) will experience lower geometries implying that downlink inter-cell interference can be more pronounced and limit the achievable bit rates. Hence, in dense cellular deployments interference mitigation techniques have the potential to substantially improve the user performance. Interference mitigation can either take place on the transmitter side or on the receiver side, or a combination of both. Interference mitigation techniques often explore the structure of the physical layer transmission of the radio access technology.
In heterogeneous networks, a mixture of cells of different sized and overlapping coverage areas is deployed. One example of such deployments is illustrated in FIG. 1, where pico cells 12 are deployed within the coverage area of a macro cell 10.
Other examples of low power nodes, also referred to as points, in heterogeneous networks are home base stations and relays. As will be discussed in the following, the large difference in output power (e.g. 46 dBm in macro cells and 30 dBm or less in pico cells) results in a different interference situation than what is seen in networks where all base stations have the same output power.
Throughout these specifications, nodes or points in a network are often referred to as being of a certain type, e.g., “macro” or “pico”. Unless explicitly stated otherwise, this should not be interpreted as an absolute quantification of the role of the node/point in the network, but rather as a convenient way of discussion the roles of different nodes/points relative each other. Thus, a discussion about macro and pico cells could, for example, also be applicable to the interaction between micro and femto cells.
One aim of deploying low power nodes, such as pico base stations, within the macro coverage area is to improve system capacity by means of cell splitting gains as well to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployments are particularly effective for covering traffic hotspots, i.e. small geographical areas with high user densities served by, for example, pico cells, and they represent an alternative deployment to denser macro networks.
The most basic means to operate heterogeneous deployments is to apply frequency separation between the different layers, i.e. macro cell 10 and pico cells 12 in FIG. 1 operate on different non-overlapping carrier frequencies and thereby avoid any interference between the layers. With no macro cell interference towards the under laid cells, here exemplified by the pico cells 12 in FIG. 1, cell splitting gains are achieved when all resources can simultaneously be used by the under laid cells. The drawback of operating layers on different carrier frequencies is that it may lead to resource-utilization inefficiency. For example, with low activities in the pico cells 12, it could be more efficient to use all carrier frequencies in the macro cell 10 and then basically switch off that of the pico. However, the split of carrier frequencies across layers is typically done in a static manner.
Another related means to operate heterogeneous networks is to share radio resources on same carrier frequencies by coordinating transmissions across macro and under laid cells. This type of coordination refers to as Inter-Cell Interference Coordination (ICIC) in which certain radio resources are allocated for the macro cells during some time period, whereas the remaining resources can be accessed by the under laid cells without interference from the macro cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the above split of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between the nodes. In LTE, an X2 interface has been specified in order to exchange different types of information between base station nodes. One example of such information exchange is that a base station can inform other base stations that it will reduce its transmit power on certain resources.
Time synchronization between base station nodes is required to ensure that ICIC across layers will work efficiently in heterogeneous networks. This is in particular of importance for time domain based ICIC schemes where resources are shared in time on the same carrier.
Physical Layer Design of LTE
The physical layer transmission in LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform spread OFDM in the uplink. The basic LTE physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 2, where each resource element 14 corresponds to one subcarrier during one OFDM symbol interval 16 including a cyclic prefix 18.
In the time domain, LTE downlink transmissions are organized into radio frames 20 of 10 ms, each radio frame 20 consisting of ten equally-sized subframes 22 of 1 ms, as illustrated in FIG. 3. A subframe 22 is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of Resource Blocks (RB), where an RB corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive-in-time RBs represent an RB pair and correspond to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where the base station (also referred to as eNodeB) transmits downlink assignments/uplink grants to certain UEs via the Physical Downlink Control CHannel (PDCCH), or the enhanced PDCCH (ePDCCH) introduced in LTE Rel.11. In the LTE downlink, data is carried by the Physical Downlink Shared CHannel (PDSCH). In the uplink, the corresponding data channel is referred to as the Physical Uplink Shared CHannel (PUSCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and span (more or less) the whole system bandwidth, whereas ePDCCH is mapped on RBs within the same resource region as used for PDSCH. Hence, ePDCCHs are multiplexed in the frequency domain with the PDSCH and it may be allocated anywhere over the entire subframe. A UE that has decoded an assignment carried by a PDCCH, or ePDCCH, knows which resource elements in the subframe contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows the time/frequency resources upon which it should transmit.
Demodulation of sent data requires estimation of the radio channel, which is done by using transmitted Reference Signals (RS), i.e., symbols that are known by the receiver. In LTE, Cell-specific Reference Signals (CRS) are transmitted in all downlink subframes and in addition to assist downlink channel estimation they are also used for mobility measurements performed by the UEs. LTE also supports UE specific RS, i.e. DeModulation Reference Signals (DMRS), which are provided to assist channel estimation for demodulation purposes only.
FIG. 4 illustrates how the mapping of PDCCH 26 and PDSCH 28 and CRS 30 can be done on resource elements within a downlink subframe 24. In this example, the PDCCHs 26 occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data carried by PDSCH 28 can start at the second OFDM symbol. Since the CRS 30 is common to all UEs in the cell, the transmission of CRS 30 cannot be easily adapted to suit the needs of a particular UE. This is in contrast to DMRS, which means that each UE has reference signals of its own placed in the data region 32 of FIG. 4, as part of PDSCH. In LTE, subframes can be configured as Multimedia Broadcast Single Frequency Network (MBSFN) subframes, in which case CRSs are only present in the PDCCH 26 control region 34.
The length of the PDCCH control region, which can vary on a subframe-to-subframe basis, is conveyed in the Physical Control Format Indicator CHannel (PCFICH). The PCFICH is transmitted within this control region, at locations known to the UEs. After a UE has decoded the PCFICH, it knows the size of the control region and in which OFDM symbol the data transmission starts. Also transmitted in the control region is the Physical Hybrid-ARQ Indicator CHannel (PHICH). This channel carries ACKnowledgement/Negative ACKnowledgement (ACK/NACK) responses to a UE to inform the UE of whether the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
FIG. 5 illustrates a downlink subframe showing 10 RB pairs 36 and configuration of three ePDCCH regions 38 of size 1 Physical Resource Block (PRB) pair each. The remaining RB pairs can be used for PDSCH transmissions.
In LTE Rel.11, it has been agreed to introduce UE-specific transmission for control information in the form of enhanced control channels, by allowing the transmission of generic control messages to a UE based on UE-specific reference signals. These enhanced control channels are placed in the data region, as shown in FIG. 5. In LTE Rel.11, for example, the enhanced PDCCH (ePDCCH) has been introduced. Enhanced PHICH (ePHICH) may be introduced in later releases. For the enhanced control channel in Rel.11, it has been agreed to use antenna port p ∈{107,108,109,110} for demodulation.
FIG. 6 illustrates an example of UE-specific reference symbols used for ePDCCH in LTE. R7 and R9 represent the DeModulation Reference Signal (DMRS) corresponding to antenna port 107 and 109 respectively. In addition, antenna port 108 and 100 can be obtained by applying an orthogonal cover as (1,−1) over adjacent pairs of R7 and R9 respectively. Even numbered slots 40 and odd numbered slots 42 are indicated by intervals in FIG. 6.
This enhancement means that precoding gains can be achieved for control channels, as well as for data channels. Another benefit is that different PRB pairs (or enhanced control regions) can be allocated to different cells or different transmission points within a cell, thus facilitating inter-cell or inter-point interference coordination between control channels. This is especially useful for the heterogeneous scenario, as will be discussed in the next section.
As previously indicated, CRS are not the only reference symbols available in LTE. As of LTE Release-10, a new RS concept was introduced with separate UE-specific RS for demodulation of PDSCH and RS for measuring the channel for the purpose of CSI feedback from the UE. The latter is referred to as CSI-RS. CSI-RS are not transmitted in every subframe, and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to a Radio Resource Control (RRC) configured periodicity parameter and an RRC-configured subframe offset.
A UE operating in connected mode can be requested by the base station to perform CSI reporting, e.g., reporting a suitable Rank Indicator (RI), one or more Precoding Matrix Indicators (PMIs) and a Channel Quality Indicator (COI). Other types of CSI are also conceivable, including explicit channel feedback and interference covariance feedback, but are not yet supported in the standard. The CSI feedback assists the base station in scheduling, including deciding the subframe and RBs for the transmission and which transmission scheme/precoder to use. The CSI feedback also provides information on a proper user bit rate for the transmission (link adaptation). In LTE, both periodic and aperiodic CSI reporting is supported. In the case of periodic CSI reporting, the terminal reports the CSI measurements on a configured periodical time basis on the physical uplink control signaling (PUCCH), whereas with aperiodic reporting the CSI feedback is transmitted on the physical uplink shared channel (PUSCH) at pre-specified time instants after receiving the CSI grant from the base station. With aperiodic CSI reports, the base station can thus request CSI reflecting downlink radio conditions in a particular subframe. A multitude of feedback modes are available. The eNodeB can configure the UE to report according to one feedback mode on PUSCH and another on PUCCH. The aperiodic modes on PUSCH are referred to as PUSCH 1-2, 2-0, 2-2, 3-0, 3-1, respectively and the periodic ones on PUCCH as 1-0, 1-1, 2-0, 2-1, respectively. These are explained in 3GPP TS 36.213 V11.3.0 (“Physical Layer Procedures”).
A detailed illustration of which resource elements within a resource block pair that may potentially be occupied by the new UE-specific RS and CSI-RS is provided in FIG. 7. The CSI-RS utilize an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As seen, many different CSI-RS patterns are available. For the case of 2 CSI-RS antenna ports 44, we see that there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 CSI-RS antenna ports 46 and 8 CSI-RS antenna ports 48, respectively. For Time Division Duplex, some additional CSI-RS patterns are available. In FIG. 7, potential positions are indicated for CRS port#1 and #2 50, CRS port #3 and #4 52, DMRS (Rel-8) port#5 (if configured) 54, UE specific RS or DMRS (Rel-9/10) 56, PDCCH 58, PDSCH 60, and CSI-RS (which are marked with a number corresponding to the CSI-RS antenna port).
In the following discussion, the term CSI-RS resource may be mentioned. In such a case, a resource corresponds to a particular pattern present in a particular subframe. Thus, two different patterns in the same subframe or the same CSI-RS pattern but in different subframes in both cases constitute two separate CSI-RS resources.
The CSI-RS patterns may also correspond to so-called zero-power CSI-RS, also referred to as muted REs. Zero-power CSI-RS correspond to a CSI-RS pattern whose REs are silent, i.e., there is no transmitted signal on those REs. Such silent patterns are configured with a resolution corresponding to the 4 antenna port CSI-RS patterns. Hence, the smallest unit to silence corresponds to four REs.
The purpose of zero-power CSI-RS is to raise the SINR for CSI-RS in a cell by configuring zero-power CSI-RS in interfering cells so that the REs otherwise causing the interference are silent. Thus, a CSI-RS pattern in a certain cell is matched with a corresponding zero-power CSI-RS pattern in interfering cells. Raising the SINR level for CSI-RS measurements is particularly important in applications such as coordinated multi-point (CoMP) or in heterogeneous deployments. In CoMP, the UE is likely to need to measure the channel from non-serving cells and interference from the much stronger serving cell would in that case be devastating. Zero-power CSI-RS is also needed in heterogeneous deployments where zero-power CSI-RS in the macro-layer is configured so that it coincides with CSI-RS transmissions in the pico-layer. This avoids strong interference from macro nodes when UEs measure the channel to a pico node.
The PDSCH is mapped around the Resource Elements (Res) occupied by CSI-RS and zero-power CSI-RS, so it is important that both the network and the UE are assuming the same CSI-RS/zero power CSI-RS configuration or else the UE is unable to decode the PDSCH in subframes containing CSI-RS or their zero-power counterparts.
Before an LTE terminal can communicate with an LTE network it first has to find and acquire synchronization to a cell within the network, i.e., performing cell search. Then it has to receive and decode system information needed to communicate with and operate properly within the cell, and finally access the cell by means of the so-called random-access procedure.
In order to support mobility, a terminal needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell and neighbor cells. The reception quality of the neighbor cells, in relation to the reception quality of the current cell, is then evaluated in order to conclude whether a handover (for terminals in connected mode) or cell re-selection (for terminals in idle mode) should be carried out. For terminals in connected mode, the handover decision is taken by the network based on measurement reports provided by the terminals. Examples of such reports are Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). Depending on how these measurements, possibly complemented by a configurable offset, are used, the UE can for example be connected to the cell with the strongest received power, or the cell with the best path gain, or something between the two. These selection strategies do not result in the same selected cell, as the base station output powers of cells of different type are different. This is sometimes referred to as link imbalance.
For example, the output power of a pico base station or a relay is in the order of 30 dBm or less, while a macro base station can have an output power of 46 dBm. Consequently, even in the proximity of the pico cell 62, the downlink signal strength from the macro cell 64 can be larger than that of the pico cell 62. From a downlink perspective, it is often better to select cell based on downlink received power, whereas from an uplink perspective, it would be better to select cell based on the path loss. The cell selection approaches are illustrated in FIG. 8 illustrating uplink and downlink coverage in a mixed cell scenario.
Hence, in the above scenario, it might be a better case, from a system perspective, to connect to the pico cell 62 even if the macro downlink is much stronger than the pico cell downlink. However, ICIC across layers would be needed when terminals operate within the region of the UL border 66 and the DL border 68 (the link imbalance zone) depicted in FIG. 8. Some form of interference coordination across the cell layers is especially important for the downlink control signaling. If this interference situation is not handled appropriately, a terminal in the region between the DL border 66 and UL border 68 in FIG. 8 and connected to the pico cell 62 cannot receive the downlink control signaling from the pico.
One approach of providing ICIC across layers is illustrated in FIG. 9, where an interfering macro cell 72 (downlink interference towards a pico cell 74) avoids scheduling unicast traffic in certain subframes 70, implying that neither PDCCHs nor PDSCH occur in those subframes 70. In such a way, it is possible to create low interference subframes, which can be used to protect pico users operating in the link imbalance zone. The macro base station (MeNB) indicates via the backhaul interface X2 to the pico base station (PeNB) which subframes it will avoid scheduling users within. The PeNB can then take this information into account when scheduling users operating within the link imbalance zone; such that these users are scheduled in subframes aligned with the low interference subframes at the macro layer, i.e. in interference protected subframes. However, pico cell users operating within the DL border can be scheduled in all subframes, i.e. in both protected and non-protected subframes.
In principle, data transmission (but not control signaling) in different layers could also be separated in the frequency domain by ensuring that scheduling decisions in the two cell layers are non-overlapping in the frequency domain, e.g. by exchanging coordination messages between the different base stations. For the control signaling this is not possible, as it must span the full bandwidth, according to the LTE specifications, and hence a time-domain approach must be used.
Coordinated Multipoint Transmission
Coordinated Multipoint Transmission (CoMP) transmission and reception refers to a system where the transmission and/or reception at multiple, geographically separated antenna sites is coordinated in order to improve system performance. More specifically, CoMP refers to coordination of antenna arrays that have different geographical coverage areas and/or coverage areas that are covered in different ways. In the subsequent discussion we refer to an antenna covering a certain geographical area in a certain manner as a point, or more specifically as a Transmission Point (TP). The coordination can either be distributed, by means of direct communication between the different sites, or by means of a central coordinating node.
CoMP is a tool introduced in LTE to improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput. In particular, the goal is to distribute the user perceived performance more evenly in the network by taking control of the interference in the system, either by reducing the interference and/or by better prediction of the interference.
CoMP operation targets many different deployments, including coordination between sites and sectors in cellular macro deployments, as well as different configurations of Heterogeneous deployments, where for instance a macro node coordinates the transmission with pico nodes within the macro coverage area.
There are many different CoMP transmission schemes that are considered; for example,                Dynamic Point Blanking where multiple transmission points coordinates the transmission so that neighboring transmission points may mute the transmissions on the TFREs that are allocated to UEs that experience significant interference.        Dynamic Point Selection where the data transmission to a UE may switch dynamically (in time and frequency) between different transmission points, so that the transmission points are fully utilized.        Coordinated Beamforming where the TPs coordinate the transmissions in the spatial domain by beamforming the transmission power in such a way that the interference to UEs served by neighboring TPs are suppressed.        Joint Transmission (JT) where the signal to a UE is simultaneously transmitted from multiple TPs on the same time/frequency resource. The aim of joint transmission is to increase the received signal power and/or reduce the received interference (if the cooperating TPs otherwise would serve some other UEs without taking our JT UE into consideration).        
The concept of a “point” is heavily used in conjunction with techniques for CoMP. In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Thus a point might correspond to one of the sectors at a site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or having antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points. Downlink (DL) CoMP operations may include, e.g., serving a certain UE from multiple points, either at different time instances or for a given subframe, on overlapping or not overlapping parts of the spectrum. Dynamic switching between transmission points serving a certain UE is often termed as Dynamic Point Selection (DPS). Simultaneously serving a UE from multiple points on overlapping resources is often termed as JT. The point selection may be based, e.g., on instantaneous conditions of the channels, interference or traffic. CoMP operations are intended to be performed, e.g., for data (PDSCH) channels and/or control channels such as ePDCCH.
CoMP Feedback
A common denominator for the CoMP transmission schemes is that the network needs CSI information not only for the serving TP, but also for the channels linking the neighboring TPs to a terminal. By, for example, configuring a unique CSI-RS resource per TP, a UE can resolve the effective channels for each TP by measurements on the corresponding CSI-RS. A CSI-RS resource can loosely be described as the pattern of resource elements on which a particular CSI-RS configuration is transmitted. A CSI-RS resource is determined by a combination of “resourceConfig”, “subframeConfig”, and “antennaPortsCount”, which are configured by RRC signaling. It should be noted that the UE is likely unaware of the physical presence of a particular TP, it is only configured to measure on a particular CSI-RS resource, without knowing of any association between the CSI-RS resource and a TP.
CoMP feedback for LTE Rel 11 builds upon per CSI-RS resource feedback which corresponds to separate reporting of CSI for each of a set of CSI-RS resources. Such a CSI report could for example correspond to a PMI, RI, and/or CQI, which represent a recommended configuration for a hypothetical downlink transmission over the same antennas used for the associated CSI-RS (or as the RS used for the channel measurement). More generally, the recommended transmission should be mapped to physical antennas in the same way as the reference symbols used for the CSI channel measurement. Potentially, there could be interdependencies between the CSI reports; for example, they could be constrained to have the same RI, so-called rank inheritance.
Typically there is a one-to-one mapping between a CSI-RS and a TP, in which case per CSI-RS resource feedback corresponds to per-TP feedback; that is, a separate PMI/RI/CQI is reported for each TP.
The considered CSI-RS resources are configured by the eNodeB as the CoMP Measurement Set.
Interference Measurements for CoMP
For efficient CoMP operation it is equally important to capture appropriate interference assumptions when determining the CQls as it is to capture the appropriate received desired signal.
In uncoordinated systems the UE can effectively measure the interference observed from all other TPs (or all other cells), which will be the relevant interference level in an upcoming data transmission. Such interference measurements are typically in releases prior to Rel-11 performed by analyzing the residual interference on CRS resources (after the UE subtracts the impact of the CRS signal).
In coordinated systems performing CoMP such interference measurements becomes increasingly irrelevant. Most notably, within a coordination cluster an eNodeB can to a large extent control which TPs that interferes a UE in any particular TFRE. Hence, there will be multiple interference hypotheses depending on which TPs are transmitting data to other terminals.
Interference Measurement Resource
For the purpose of improved interference measurements, new functionality is introduced in LTE Release 11, where the agreement is that the network will be able to configure a UE to measure interference on a particular Interference Measurement Resource (IMR) that identifies a particular set of REs in the time and frequency grid that is to be used for a corresponding interference measurement. An alternative name of IMR used in LTE specifications is CSI-Interference Measurement (CSI-IM). The network can thus control the interference seen on an IMR, by for example muting all TPs within a coordination cluster on the IMR, in which case the UE will effectively measure the inter CoMP cluster interference. Moreover, it is essential that an eNodeB can accurately evaluate the performance of a UE given different CoMP transmission hypotheses—otherwise the dynamic coordination becomes meaningless. Thus the system need to be able to track/estimate also different intra-cluster interference levels corresponding to different transmission and blanking hypotheses.
Quasi-Co-Location of Antenna Ports
One fundamental property of DL CoMP is the possibility to transmit different signals and/or channels from different geographical locations (points). One of the principles guiding the design of the LTE system is transparency of the network to the UE. In other words, the UE is able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs or network deployments.
Channel estimation based on RS often makes use of assumptions regarding similarity of the channels over which different RS (where each RS typically corresponds to a logical entity called antenna port) is transmitted. Such assumptions of similar channel properties between different antenna ports are referred to as antenna port quasi-co-location assumptions. The overall co-location assumptions a UE makes for a certain channel type (e.g. for PDSCH, or for ePDCCH) are collected into a co-location UE behavior, or “behavior” for short. The “quasi” part of quasi-co-location refers to the fact that co-location does not necessarily imply physical colocation of the antenna ports associated to the channels, but rather colocation with respect to the listed channel and signal properties.
It is observed here that, even though in general the channel from each antenna port to each UE receive port is substantially unique, some statistical properties and propagation parameters may be common or similar among different antenna port, depending on whether the different antenna ports originate from the same point or not. Such properties include, e.g., the received power level for each port, the delay spread, the Doppler spread, the received timing (i.e., the timing of the first significant channel tap) and the frequency shift.
Typically, channel estimation algorithms perform a three step operation. A first step consists of the estimation of some of the statistical properties of the channel. A second step consists of generating an estimation filter based on such parameters. A third step consists of applying the estimation filter to the received signal in order to obtain channel estimates. The filter may be equivalently applied in the time or frequency domain. Some channel estimator implementations may not be explicitly based on the three steps method described above, but still exploit the same principles.
Obviously, accurate estimation of the filter parameters in the first step leads to improved channel estimation. Even though it is often in principle possible for the UE to obtain such filter parameters from observation of the channel over a single subframe and for one RS port, it is usually possible for the UE to improve the filter parameters estimation accuracy by combining measurements associated with different antenna ports (i.e., different RS transmissions) sharing similar statistical properties. Furthermore, the channel estimation accuracy may be improved by combining RSs associated to multiple PRBs.
It is observed here that the network is typically aware of which RS ports are associated with channels with similar properties, based on its knowledge of how antenna ports are mapped to physical points, while the UE is conventionally not aware a priori of such information, because of the transparency principle of network transmission. This creates a need to introduce antenna port quasi-co-location assumptions in the LTE specifications to firmly establish which antenna ports the UE may assume to have similar properties and what those properties are. For example, the new transmission mode 10 introduced in Rel-11 supports dynamic signaling of quasi-co-location information using a DCI format transmitted on a DL control channel (like PDCCH or ePDCCH). For example, Downlink Control Information (DCI) format 2D used in transmission mode 10 may be used for signaling that DMRS for PDSCH is co-located with a specific CSI-RS resource and a specific CRS. Basically, a message state in the DCI format gives an index into a configurable table of CSI-RS resources used for defining the meaning of the message state.
The same message state is also used to signal information on how to map the PDSCH onto the resource element grid, including what OFDM symbol to start the PDSCH on, which REs corresponding to a CRS to map around, what MBSFN configuration to assume, and what Zero Power (ZP) CSI-RS configuration to assume. The RRC-configurable table defining the meaning of each associated message is popularly referred to as the PQI table, where PQI stands for PDSCH mapping and Quasi-co-location Information. Correspondingly, the message state itself may be referred to as a PQI indicator.
Interference Mitigation Aspects
Interference mitigation on the transmitter side refers to methods that aim to coordinate the physical channel transmissions across cells to avoid severe interference. A simple example is when an aggressor base station occasionally mutes its transmissions on certain radio resources in order for a victim cell to schedule interference sensitive UEs on radio resources with reduced interference. LTE features to coordinate transmissions have been specified in the context of ICIC and CoMP. In the case of ICIC, an eNodeB (eNB) sends a message over the LTE inter-eNB interface X2 with coordination information that a receiving eNB can take into account when scheduling interference sensitive users. In the case of CoMP, a cluster of transmission points, or base stations, can jointly and synchronously transmit the same signals to a UE and by then increase the received power on the desired signals or it can as in the ICIC case coordinate the transmissions to avoid inter-point interference.
The following ICIC messages over X2 have been specified in 3GPP TS 36.423 V11.8.0 “X2 Application Protocol (X2AP)”:                UL Overload Interference Indication (OII) indicates per RB the interference level (low, medium, high) experienced by the indicated cell on all RBs.        UL High Interference Indication (HII) indicates per RB the occurrence of high interference sensitivity, as seen from the sending eNB.        Received Narrow Transmit Power (RNTP) indicates per RB whether DL transmission power is lower than the value indicated by a threshold.        Almost Blank Subframe (ABS) pattern indicating the subframes the sending eNB will reduce power on some physical channels and/or reduced activity.        
The X2 messages OII, HII and RNTP were specified in LTE Rel.8 and represent methods for coordinating physical data channel transmissions in the frequency domain across cells. The ABS message, however, was specified in LTE Rel.10 as a time domain mechanism to primarily protect reception of PDCCH, PHICH and PDSCH in the small cells by letting macro cells occasionally mute, or reduce transmits power on PDCCH/PDSCH in certain subframes. The eNB ensures backwards compatibility towards UEs by still transmitting necessary channels and signals in the ABS for acquiring system information and time synchronization.
On the receiver side of techniques to mitigate inter-cell interference, Interference Rejection Combining (IRC) is a well-known receiver type for suppressing interference and requires estimation of an interference/noise covariance matrix. Other receiver types for interference mitigation are those based on Interference Cancellation (IC), in which unwanted received signals (intra/inter-cell interference) are estimated and subtracted from the received signals. Both IRC and IC are, since 3GPP Rel.11, established as UE reference receiver techniques in LTE. However, IC in LTE Rel.11 was restricted to cancellation of always-on signals, such as the CRS, in which the network assists the UE on how these signals are transmitted in the aggressor cells. There is currently an ongoing LTE Rel.12 study on network-assisted interference cancellation and suppression of interference corresponding to scheduling of data.
Inter-cell interference is often one of the dominant impairments limiting receiver performance and the achievable data rates in cellular networks. In traditional, linear receivers, multiple antennas, spatial selectivity and IRC weight design has been used to mitigate such interference.
More advanced receivers employing enhanced interference suppression schemes, maximum likelihood techniques and IC techniques are gaining popularity for mitigating DL interference arising from neighbor-cell transmissions to UEs in those cells. Such receivers may be used to explicitly remove all or parts of the interfering signal. As an example, an IC receiver in the victim UE may be used to demodulate and optionally decode the interfering signals, producing an estimate of the transmitted and the corresponding received signal, and removing that estimate from the total received signal to improve the effective Signal to Interference plus Noise Ratio (SINR) for the desired signal. In post-decoding IC receivers, the interfering data signal is demodulated, decoded, its estimated contribution to the received signal is regenerated, and subtracted. In pre-decoding receivers, the regeneration step is performed directly after demodulation, bypassing the channel decoder. The preferred mode to perform such cancellation is by applying soft signal mapping and regeneration, as opposed to hard symbol or bit decisions. ML receivers can as well be used to jointly detect the useful and interference signals in accordance to the ML criterion. Additionally iterative ML receivers can be defined which exploits the decoding of the interfering signal(s).
To apply these advanced receivers to signals originating from other cells, knowledge of certain signal format parameters may be required to configure the receiver. For pre-decoding IC, the resource allocation, modulation format, any precoding applied, the number of layers, etc., may be useful, and may be obtained via blind estimation, eavesdropping other-cell control signaling, or via network (NW) assistance features. For post-decoding, additional transport format parameters are required, which may typically only be obtained from receiving or eavesdropping the related control signaling.
The two approaches differ by the achievable cancellation efficiency, i.e., the fraction of the impairment power left after the cancellation operation—they may be essentially equal in some scenarios and vary significantly in others, the post-decoding IC approach typically providing superior performance at “high” SINR operating points”, i.e. when there is high probability to decode the interfering signal. They typically also differ by the computational resources required (the post-decoding solution implies Turbo decoding processing) and by the processing delay incurred (the post-decoding solution requires buffering the entire code block of the interfering signal).
Network-Assisted Interference Cancellation and Suppression
In 3GPP, interference cancellation has been widely discussed. In Rel-11, CRS-IC, Primary Synchronization Sequence (PSS)/Secondary Synchronization Sequence (SSS), and Physical Broadcast CHannel (PBCH) IC has been standardized for heterogeneous network deployments. To enhance UE performance, PDSCH and PDCCH/ePDCCH cancellation are under discussion in Rel-12. In Rel-11, in order to enable CRS-IC, PSS/SSS, and PBCH IC, the eNB needs to provide certain assistance information, including CRS ports, cell ID, and MBSFN configuration. The UE utilizes this information to cancel CRS, PSS/SSS, and PBCH. In Rel-12, how to enhance UE performance by interference cancellation at data channels is still under discussion.
As for terminology in this disclosure, we use                Serving Cell (SC), which is the cell to which the UE is currently attached        Neighboring Cell(s) (NC), which is the cell(s) where the transmission of data is typically interfering with the reception of data from the SC        Interference Cancellation (previously denoted IC), which is the regeneration and subtraction of interfering data or control signaling from the desired received signal        
For network-assisted interference cancellation and suppression, different interference mitigation methods can be used. Two kinds of interference cancellation methods are extensively discussed. One is Symbol Level Interference Cancellation (SLIC), and the other is CodeWord level Interference Cancellation (CWIC). For symbol level interference cancellation, the interference signal is regenerated after demodulation and further subtracted from the receiving signal. For codeword level interference cancellation, the interference signal is synthesized after channel decoding, and further subtracted from the receiving signal. The main interference suppression method that has been discussed is Enhanced-IRC, which is an IRC receiver where the interference covariance matrix is parametrically built. There is also a fourth interference mitigation algorithm that has been studied, which is the Maximum Likelihood (ML) receiver, where the best modulation symbol is found according to given interference distribution.
Common for any kind of effective IC algorithm is that it is a soft IC. This means that it will take the certainty of a certain symbol value, or parameter value into account when determining how to regenerate and cancel the transmitted information. For example, when there is a lot of interference or noise on the data stream to be cancelled, the quality of demodulation and/or decoding can be expected to be low, and then typically the regenerated data symbols are created with lower energy to only cancel the certain part of the symbol and not introduce a lot of additional errors. If this is done correctly, then soft IC should never introduce additional errors or interference in the cancellation step. Unfortunately, as discussed under the “Problems with Existing Solutions”, there are always cases where the soft IC algorithm is unaware of uncertainty or parameter errors and therefore cannot avoid errors. The soft IC is a generalization of the hard IC, where the symbol regeneration is only to a set of fixed values. In the following discussion we will only use the more general soft IC, but it can easily be translated also to the hard IC case. The ML receiver is another kind of hard decision interference mitigation algorithm that also has a soft counterpart. For ML the hard decision is made over several layers at once. Similar arguments discussed for soft IC also hold for ML receiver, hence in the following discussion about drawbacks for soft IC will also hold for ML receiver. CRS IC is a specific version of hard IC, where the regenerated symbol is only regenerated to already the known symbol value of the pilot.
To facilitate the interference estimation at UE sides, firstly, the network provides to the UEs information about transmission properties of interfering signals, so that the UEs can estimate channel status of the interferers that are intended to be cancelled. Secondly, depending on the UE's capability, the UE may need to know the interfering signals' structure, such as modulation style/feature (for instance, modulation order). The more information that is known regarding this interference signal structure, including the consistency of the structure during a size of scheduling resource granularity, the better the UE can efficiently estimate and synthesize the interfering signal with a low complexity, which is a critical factor for standardization and product's business value.
In short, network assistance is preferably to provide information about the interferers, including                any information aiding the UEs to infer interfering channel status and        Interfering signal structure or features.        
Network assistance always comes with a cost to the system, though, in that it consumes valuable system resource when being transmitted to the receiver. Therefore, it is expected that a NAICS receiver will have to blindly detect quite a few of the NC parameters, often based on statistics on the received data signal.
In the following, a NAICS receiver is defined as a receiver that is at least capable of mitigating or suppressing PDSCH interference and cancelling CRS interference. Mitigation or cancellation of different types of interference is not precluded. A legacy Rel-11 receiver could be the “baseline receiver” which has been considered for the definition of the performance requirements (see type A performance requirements in 3GPP TS 36.101 V11.8.0 “User Equipment (UE) radio transmission and reception”, i.e., Linear MMSE-IRC). Note that in the following, a legacy receiver need not be exactly this baseline receiver, but is a receiver that does not include the principal features of NAICS, i.e. estimating each modulation symbol of interfering PDSCHs and/or channel estimation of individual interferers.
CQI Computation for NAICS
A UE is expected to derive and report CQI to the network with the highest modulation and coding scheme that the UE can guarantee with 10% Block Error Rate (BLER) on the CQI reference resource for the transport format corresponding to the CQI. Hence, when the UE derives CQI it takes into account the channel quality and the receiver capability. Thus, a more advanced receiver is expected to be able to report higher CQI index (higher modulation or higher coding rate) than a less advanced receiver, under the same channel conditions. For a NAICS capable UE, this implies that its mitigation efficiency under radio conditions in the CQI reference resource needs to be determined, in order to be compliant with current CQI definition.
In the following, pre-NAICS CQI corresponds to the case when the CQI is derived based without assuming NAICS functionality (e.g., on the legacy Rel-11 receiver). Post-NAICS CQI corresponds to the case when the CQI is derived by taking the NAICS functionality into account.
Problems with Existing Solutions
A Rel-12 UE could support advanced receivers that allow mitigating or cancelling of the inter-cell or intra-cell interference. It is likely that assistance information will be optionally provided in order to facilitate the advanced receiver operation, in order to limit the complexity of the cancellation/mitigation operation, and in particular of the blind detection of the all the parameters required by NAICS receivers. However, it is not mandated for the network to restrict its behavior to follow the assistance information. When the network does not provide the assistance information or when it does not follow the behavior as defined by the same assistance information the network does not know whether and when the UE applies the advanced receiver.
Assistance information may include Cell ID, PA, PB parameters, MBSFN configurations, etc., of the candidate interfering cells (PA and PB are described in 3GPP TS 36.213 V11.3.0 (“Physical Layer Procedures”), Section 5.2.). In addition, it has been recently agreed that the UE may assume the following “NAICS favorable” conditions whenever it applies NAICS functionality, i.e.:                The cells are all synchronized.        The same system bandwidth is used in all the cells.        The same Cyclic Prefix (CP) is used in all the cells.        A certain minimum scheduling granularity is at least used in all the cells (1 PRB pair).        
Clearly, it is not mandated for the network to restrict its behavior to follow the above mentioned conditions, nor to necessarily follow or to provide the assistance information. When the network does not provide the assistance information, or when it does not follow the behavior as defined above, it is uncertain/unknown:                whether the UE has the capability to autonomously detect that the above conditions are not respected,        whether the UE applies the NAICS receiver, and        under which conditions the UE applies the NAICS receiver.        
Because they are often non-linear, the performance of NAICS receivers can vary considerably more with interference characteristics than that of legacy receivers. Consequently, it is unlikely that NAICS receivers will be tested in all relevant combinations of interference and desired signal characteristics (such as modulation state, rank, received signal power, transmission mode, etc.) where performance can substantially vary. Therefore, it is difficult to guarantee that a NAICS UE will have a performance at least as good as a legacy UE. If a NAICS UE continuously assumes that network conditions are suitable for NAICS reception when in fact the network conditions are unsuitable, then the UE will likely have to perform both decodes continuously. Such redundant decoding can increase UE complexity and/or require extra processing that increases current drain on the UE's battery.
It should be noted also that it currently is not clear how channel state feedback information will be defined in the context of NAICS. In case only pre-NAICS CQI feedback is considered in the context of NAICS, it will be assumed that the network can compensate the suboptimal UE CQI feedback by re-optimizing link adaptation using the Outer Loop Link Adaptation (OLLA) technique. However, the network does not have so far any knowledge/guarantee that a NAICS capable UE has enabled or disabled its NAICS receiver at any given time, which could lead to conservative scheduling decisions and hence suboptimal performance.
In transmission modes 1-9, LTE Rel-11 does not constrain how the UE derives interference measurements for computing CQI. Because interference measurements are a core part of NAICS reception and CSI calculation, this makes it difficult to differentiate between how CQI is calculated for post-NAICS and for pre-NAICS reception. As discussed above, the CQI reported with post and pre-NAICS receiver can be significantly different, and so there should be some mechanism that allows the network to know when post-NAICS CQI is reported or not, at least for transmission modes 1-9.